1. Field of the Invention
The present invention concerns a hybrid CSI procedure for a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance spectroscopy has been used for more than four decades in basic physical, chemical, and biochemical research, e.g. as an analysis technique or for determining the structure determination of complex molecules. Magnetic resonance spectroscopy, like magnetic resonance tomography, is based on the principle of nuclear magnetic resonance, but the objective of spectroscopy is not imaging, but the analysis of a substance. Resonance frequencies of isotopes that have a magnetic moment, e.g. 1H, 13C or 31P, are dependent on the chemical structure of molecules, in which the aforementioned isotopes are bonded. A determination of the resonance frequencies therefore allows a differentiation between different substances. The signal intensity at the different resonant frequencies provides information about the concentration of the molecule in question.
If a molecule is disposed in the main magnetic field of a magnetic resonance device, as occurs in spectroscopy, electrons of the molecule protect the atomic nuclei of the molecule from the main magnetic field. Due to this effect, the local magnetic field at the location of an atomic nucleus is modified by a factor of several millionths of the outer main magnetic field. The associated variation of the resonant frequency of this atomic nucleus is called chemical shift. Molecules are identified based on their chemical shift. Since frequency differences are easier to measure and more exactly ascertainable than absolute frequencies, the chemical shift relative to a reference signal, for example relative to the operating frequency of the magnetic resonance device, is given in ppm (parts per million).
A resonance line of an atomic nucleus can be split into several lines, if other atomic nuclei with a magnetic moment are located in the vicinity of the atomic nucleus under consideration. The cause lies in the so-called spin-spin coupling between the atomic nuclei. The magnetic flux density of the main magnetic field that acts upon an atomic nucleus thus depends not only on the electron shell around this atomic nucleus, but also on the orientation of the magnetic field of the neighboring atoms.
Clinical magnetic resonance spectroscopy is understood to be magnetic resonance spectroscopy using clinical magnetic resonance devices. The procedures of localized magnetic resonance spectroscopy differ from those of magnetic resonance imaging mainly in that, with spectroscopy, the chemical shift is resolved in addition to local tomographic resolution. Two localization procedures currently dominate clinical use. One procedure involves single volume techniques based on echo procedures, in which a spectrum of a previously selected target volume is recorded. The others are spectroscopic imaging procedures, so-called CSI procedures (Chemical Shift Imaging), which enable the simultaneous recording of spectra primarily of target volumes that are spatially connected.
Spectroscopic imaging procedures are used in clinical phosphorous as well as proton spectroscopy. A three-dimensional CSI procedure includes, for example, after a non-slice selective 90° RF pulse activating, a combination of magnetic phase coding gradients of the three spatial directions for a set period of time followed by readout of the magnetic resonance signal in the absence of all gradients. This is repeated with other combinations of phase coding gradients until the desired local resolution is reached. A four-dimensional Fourier transformation of the magnetic resonance signal delivers the desired spatial distribution of the resonance lines. A two-dimensional CSI procedure is created from the previously described three-dimensional one by replacing the aforementioned, non-slice-selective RF pulse is with a slice-selective stimulus, formed by a slice-selective RF pulse and the corresponding magnetic gradient, and a gradient in the phase-coding direction is omitted.
The normally used single volume techniques are based on detecting a stimulated echo or a secondary spin echo. In both cases, a local resolution occurs by consecutive selective stimuli of three orthogonal slices. A target volume thus is defined by an intersection volume of the aforementioned three slices. Only a magnetization of the target volume responds to all three selective RF pulses and thus contributes to the stimulated echo or secondary spin echo. The spectrum of the target volume is determined by one-dimensional Fourier transformation of the time signal corresponding to the stimulated echo or the secondary spin echo.
A hybrid CSI procedure is achieved by the integration of phase coding tables into a single volume technique. Compared to the single volume technique, the hybrid CSI procedure within a volume of interest (VOI) enables the selection of voxels. Since in slice selection the exact slice position depends on the resonant frequency of the stimulated spin ensemble, the area in which, e.g., fat is stimulated is shifted to the area in which water is stimulated. This phenomenon is called the chemical shift artifact. Some of the hybrid CSI procedures offer an adjustable parameter called frequency shift, with which it is possible to determine to which frequency the slice positioning should relate. For the selected frequency, the slice positioning is exact. With an increasing frequency spread, the shift increases linearly. Thus, with hybrid CSI procedures, it should be expected that magnetic resonance signals, which lie outside the selected frequency, can be absorbed in voxels in the border area of the volume of interest.
German OS 199 06 859 describes the combination of a volume stimulus, as with the single volume technique based on a secondary spin echo, with very selective saturation pulses outside of the volume for suppressing a signal from outside of the volume in question and for minimizing the chemical shift error in this volume.
Furthermore, the intensive water signals often are suppressed with clinical proton spectroscopy. A procedure for so-called water suppression is, for example, the CHESS technique, in which the nuclear spins of the water molecules are selectively stimulated first by narrowband 90° RF pulses and their cross-magnetization is then de-phased by the switching of magnetic field gradients. Thus, ideally, there is no further detectable magnetization of the water molecules in an immediately subsequent spectroscopy procedure